Switches and switch assemblies are known and are generally used to indicate the difference in, or move between, two states. Switches may be used to indicate positional changes, such as the change in a position of an object being monitored.
There are several situations in which switches may be used in aircraft, and in particular in aircraft landing gear. The landing gear in aircraft, and in particular any changes in state of the landing gear, are monitored very careful to ensure the safety of the aircraft when in operation. Indicating means may be used to show a change in position of two components of the aircraft landing gear relative to each other. Indicating means may also be used to show a change in state of a specific component relative to its normal operational state.
Aircraft are often towed or pushed to position the aircraft when at or near the gate, and also to move aircraft around the airport. An important consideration when towing or pushing an aircraft involves preventing the aircraft steering angle from moving beyond its mechanical limits. This is referred to as over-steer. Damage may occur when a tow vehicle attempts to steer an aircraft nose landing gear beyond its mechanical limits of travel and possibly impart a high load over the strength and of the landing gear and air frame. This loading can result in deformation, fatigue, and ultimately, failure of the parts under stress. It is useful to have an indication of whether an over-steer event has occurred as the landing gear may have become damaged.
Tow vehicles can include warning systems to alert ground operators of potential over-steer, and ground support tow bars can have built-in fuses. In addition, the nose landing gear can have a fuseable tow fitting or disconnecting torque links to prevent towing torque transferring to the landing gear and airframe. These systems have problems because they rely on proper operation by the ground operations personnel and communicating faults or warnings to the aircraft operator (e.g. the pilot).
In order to ascertain when an over-steer event has occurred, a monitoring system or switch may be used that can indicate when such an event has occurred. Proximity sensors coupled to an electronic control box have been used in the past to indicate the occurrence of an over-steer event. Proximity sensors are generally more expensive than mechanical switches and also require a continuous power source, such as a battery, to operate the sensors when the aircraft is not powered. These proximity sensor can also be expensive to install, causes increased weight, due to sensors, targets and associated electronics, and creates additional points of failure that affect costs associated with maintenance and reliability. Proximity sensor systems typically require 2 or more proximity sensors, thus adding to the weight and complexity of the system.
Some large commercial aircraft use a proximity sensor based over-steer detection system. The system uses multiple proximity sensors that are mounted at the limits and detect targets that travel near the proximity sensor to indicate over-steer. Disadvantages of these systems include the inability to detect over-steer events if the system is not powered on and the requirement of a black box for conditioning and processing the proximity sensors signals that adds additional costs and must be integrated with the aircraft.
Another problem with switches that monitor for over-steer events is that tow operators may be able to reset the switch manually. If a tow operator has caused an over-steer event, the tow operator may be inclined to reset the switch so that the over-steer will not be detected to avoid being reprimanded or punished. Also, existing mechanical switches may not be practical in a high vibration landing gear environment.